On Fri, Sep 02, 2016 at 09:43:01AM -0400, Stefan Hajnoczi wrote: > On Wed, Aug 31, 2016 at 01:05:45PM -0400, Luiz Capitulino wrote: > > We need to retrieve a VM's TSC offset in order to use > > the host's TSC to merge host and guest traces. This is > > explained in detail in this thread: > > > > [Qemu-devel] [RFC] host and guest kernel trace merging > > https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg00887.html > > > > Today, the only way to retrieve a VM's TSC offset is > > by using the kvm_write_tsc_offset tracepoint. This has > > a few problems. First, the tracepoint is only emitted > > when the VM boots, which requires a reboot to get it if > > the VM is already running. Second, tracepoints are not > > supposed to be ABIs in case they need to be consumed by > > user-space tools. > > > > This commit exports a VM's TSC offset to user-space via > > debugfs. A new file called "tsc-offset" is created in > > the VM's debugfs directory. For example: > > > > /sys/kernel/debug/kvm/51696-10/tsc-offset > > > > This file contains one TSC offset per line, for each > > vCPU. For example: > > > > vcpu0: 18446742405270834952 > > vcpu1: 18446742405270834952 > > vcpu2: 18446742405270834952 > > vcpu3: 18446742405270834952 > > > > There are some important observations about this > > solution: > > > > - While all vCPUs TSC offsets should be equal for the > > cases we care about (ie. stable TSC and no write to > > the TSC MSR), I chose to follow the spec and export > > each vCPU's TSC offset (might also be helpful for > > debugging) > > > > - The TSC offset is only useful after the VM has booted > > > > - We'll probably need to export the TSC multiplier too. > > However, I've been using only the TSC offset for now. > > So, let's get this merged first and do the TSC multiplier > > as a second step > > Can TSC offset changes occur at runtime? > > One example is vcpu hotplug where the tracing tool would need to fetch > the new vcpu's TSC offset after tracing has already started. > > Another example is if QEMU or the guest change the TSC offset while > running. If the tracing tool doesn't notice this then trace events will have > incorrect timestamps. > > Stefan
Yes they can, and the interface should mention that "the user is responsible for handling races of execution" (IMO). So the workflow is: 1) User boots VM and knows the state of the VM. 2) User runs trace-cmd on the host. Is there a need to automate gathering of traces? (that is to know the state of reboots and so forth). I don't see one. In that case, the above workflow is functional. Can you add such comments to the interface Luiz (that the value read is potentially stale).
